484
IXDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 20, No. 5
“Normal Aging” of Compounded Rubber’12 R a l p h H. McKee and Harlan A. Depew COLUMBIA UNIVERSITY, NEWYORK, N. Y.,A N D
THENEWJERSEYZINCCOMPANY, PALMERTON, PA.
T
HE aging of r u b b e r Nine c o m p o u n d s were chosen f o r an investigation .of an increase of 2.8 t i m e s products is the net reof “ n o r m a l aging” u n d e r various conditions a n d a f t e r for each 10” C. On the other different times of storage. The report is divided into sultant of c h e m i c a l hand, with the storage k m three parts: (1) aging of cured test slabs stored u n d e r reactions and changes of a perature at 30” C. the acphysico-chemical nature. different a t m o s p h e r i c conditions, including wet a n d celeration of aging would be It has long been recognized d r y ; (11) aging of r u b b e r u n d e r s t r a i n ; (111) com61 times. In the former case that conditions of s t o r a g e parative aging of cured a n d u n c u r e d rubber. Tables an investigator would report have an important influence a n d curve s h e e t s selected f r o m a large mass of d a t a a r e one day of accelerated aging on the normal aging of rubber given to illustrate how these nine r u b b e r c o m p o u n d s as approximately equivalent products. Rubber products age. to 6 months’ storage; in the stored in a warm place deteril a t t e r , one day would be orate more rapidly than when stored in a cold place, and it equivalent to 2 months’ storage. probably was recognition of this fact that led to the developFrom these considerations one can readily understand ment of the Geer3 accelerated-aging test, which accelerates why investigators are unable to agree on the quantitative aging solely by increasing the temperature during storage. relation between accelerated and “normal” aging tests. InOxygen is also an important factor in the aging of rubber asmuch as conditions of accelerated aging can be controlled products. I n normal storage the oxygen concentration varies more easily than conditions of “normal aging” during the very little, and depends only on the barometric pressure. shorter time of test, acrelerated tests can probably be dupliI n specific cases, such as inner tubes, the oxygen concentra- cated more easily than natural aging tests. This alone justition may vary considerably according to the air pressure fies considerable interest in accelerated aging tests. in the tube. Recognition of this as well as the temperature As a first step in the understanding of accelerated aging factor led to the development of the Bierer-Davis4 oxygen- tests, a program was outlined to obtain information about “normal aging.” Nine compounds were chosen (Table I) bomb method of accelerated aging. Aging is also affected by moisture. Stringfield6has pointed and the aging was determined after various conditions and out the imDortance of humidity in testing, Conovere and one times of storage. ., of the authors have made recommendaiions for controlling I-Aging of Cured Test Slabs Stored u n d e r Different laboratory humidity, and Stevens’ has shown that moisture Atmospheric Conditions improves the resistance to aging of certain stocks. The test slabs were 5 by 6 by 0.15 inch (12.7 by 15.2 It is well known that conditions of strain influence the life of vulcanized rubber and that light accentuates the deteri- by 0.38 cm.) in size and were suspended from a steel rod by oration. It seems probable that fluctuations in temperature paper clips in boxes maintained a t the desired atmospheric and humidity, including freezing temperatures, may also conditions as follows: affect the life of rubber produc,ts in storage. Consequently, (1) Outdoors on the roof of the laboratory in an open box there is no such thing as normal storage and statements protected from direct light by shutters. (Figure 1) The tem-
A-Typical white tread stock B-Lower sulfur white tread stock C-Gum compound with zinc oxide for activation D-“Pure gum“ compound E-Clay compound F-Compound containing para5n +Compound with T. P. G. substituted for hexa H-Inorganic accelerated compound I-Compound containing a fine-particle-size zinc oxide
920 920 920 920 920 920 820 920 920
55 37 55 55 55 55 55 55 37
6 13 6
..6 6 .. .. 6
1260 1260 28
..
lii0 1260 1148
..
...
.. . ... ...
597
... ... ... ...
.. .. ..
...5. .. .. ..
.. .. .. .. .. .. 12 .. ..
.. .. .. .. ..
.. .. ..
64
.. ..
.... .. .. ..
1035
1260 parts of zinc oxide by weight compared with 920 parts of rubber calculates as 22.5 volumes of pigment to 100 volumes of rubber.
giving the number of months’ normal storage equivalent to one day’s accelerated aging are meaningless unless the conditions of normal storage are carefully prescribed. Consider the Geer test in its relation to “normal aging.” The accelerated test is conducted at 70” C. If the “normal aging” is conducted at 20” C., the temperature difference LS 50” C., enough to accelerate the aging 172 times on a basis Received November 23, 1927. Presented by Mr. Depew in partial fulfilment of the requirements for the degree of doctor of philosophy from Columbia University. Geer and Evans, India Rubber J., 61, 1163 (1921). 4 Bierer and Davis, IND. ENG. CHBM.,16, 711 (1924); 17, 860 (1925). 1 Ibid., 17, 833 (1925). 8 Conover and Depew, Proc. Am. Sac. Testing Materials, 87,493 (1927). 7 J . SOC. Chem. Ind., 39, 251T (1920). 1 f
perature varied from below zero (-17.8” C.) to over 100” F. (37.8” C.). The relative humidity varied from 40 per cent t o 100 per cent. The average figures for the 4 years were 48” F. (8.9” C.) and 72 per cent relative humidity. (2) Indoors in a duplicate open box in a dark room. Th: temperature varied from about 50” F. (10”C.) to 85” F. (29.4 C.) and the relative humidity varied from 25 per cent to 100 per cent. I n the winter the average humidity was lower indoors than out, and in the summer the average indoor humidity was unusually high, owing to an air-washing installation in the building. (3) Indoors in a tight box lined with sheet zinc. Air, dried by passlng through sulfuric acid, was passed through the box continuously. (4) Indoors in a tight box lined with sheet zinc. Air, humidified by bubbling through moist glass wool, was passed through the box continuously.
May, 1928
I.VDUSTRI.4L AND EhrCXiVEERlXC CHEMISTRY
l‘hr cii’eeliveneas of the drying and hun~idifieation was ~lemunatrated qualitatively by the fact that t.he iron clips holding the samples corroded through in the wet box in a short time, whereas there was practically no corrosion in the dry box. RESULTSkvn Drscussro~ OF DATA-@) The stressstrain curves made for slabs that had received 4 years of normal aging show that in most cases the rubber stored in the wet box has bhe “softest” stress-strain curve and rubber stored io t.he dry box has the “stiffest” stressstrain curve. (C.‘nrvtv 1 to 21)
4x5
The degree of surface cracking degeials on the state of cure, the higher cures showing the more severe cracking All the cures of compound D, and the two highest cures of compound C in the dry box cracked on bending, but these nieces were brittle throuehout their thickness and not onlv bn the surface. (5) Slabs that have been seed for 4 yeam and 2 month? in a wet box and then dried fo;l month ?&exposure to a dry atmosphere have a higher tensile strength by 300 to 400 pounds per square inch (21 to 28 kg. per sq. em.) than slabs that have been kept continually in a wet atmosphere. Strangely enough, this improvement seems to be independent of state of cure. (Table 11) ~
Table 11-Effect of One Month’s DryIsg at R o o m Tern erafure on Stress-Straln Propertlea of Teet Slabs Aged In a Wet Xtmosphere fComnound AI
which he eXperimentea were tightly cured-and thisseems probable-his results TOD are in complete harmony with those of the present writers. (3) The improvement in aging of the overcures in a wet atmosphere and of the undercures in a dry atmosphere is somewhat masked on the curve sheets by the superior aging that is shown by some of the compounds in the open box outdoors. A possible explanation of this surprising fact is based on the belief of many technologists that rnhher ages better in service than on the shelf. Changes in humidity with corresponding changes in moisture content, and therefore volume, and changes in temperature with resultant contraction and expansion, might work the rubber sufficiently to improve its resistance to aging. It is planned to check this theory by experimental tests. In the meantime i t will serve until some better explanation is offered. (4) The 60-minute cure of the clay compound (E) aged much better in the dry box than in the wet box. (Curves 13 and 14) Viual observation would have led to the opposite conclusion, for the surface of the slab aged in the dry box had hardened snd could easily be cracked by bending. (Figure 2) Although the surface of the test slab in the dry box was poorer than that in the wet box, the center of the slab in the dry box appeared to be better than that in the wet box. The fact that slnhs of rubber only ‘/s inch (3 mm.) thick show a marked difference in aging on the surface and in the interior is espeeially interesting. The stress-strain curves of these test slabs a,re concave to the elongation axis. This is due to minute failures in the test piece (surface breaks) as i t stretches. At first the surface carries most of the stress, hut when the surface cracks the soft center has to carry the load. This concave curve c8.n be noted in fiber compounds when the rubber-fiber bond breaks down, and in heavily loaded stocks when the rnbber-pigment bond is breaking down. The composition of the clay Btock may not have been such as to give the best results obtainable with such a stock; therefore, no direct pigment comparison can be made or is intended. The stock aged poorly, doing best in the outdoors box. Figure I-AmBnSement of ~ e Slabs ~ t i n the Opan Storage Boxes. the Slabs Being Su pmted from Me& Rods by Cligs. T%egA- Pr.9tectedfremDirecf Lia t by Opeen Shutters and a Wooden
During the first year’s storage the dienge in aright the wet box is no doubt largely a measure of the change in moisture content. In all cases, except the “pure gum” compound, where the results were irregnlar, the slaba m the wet box gained the most in weight. The samules in the dry box actuallyiost weight in a n u m b e r o eases. (Table 111) After the first year the rate of increase in weight in the wet box is less. It seems probahle that the moisture content has reached equilibrium and that further changes i n weight in both boxes are due largely to oxidation. The time required to come to equilibrium would probably have been greatly reduced if the dry or moist air had blown rapidly over the test slahs. In the inc , ~ ~ g ~ ~ doom and OUtdOOrS boxes after 4 years in Dry Atmosphere changes i n temperature and humidity caused the weights to change irregularly. Those data in Table I11 dedmg with the change in weight of the slahs in the wet and dry boxes have been recalculated (Table IV) to show the difference in the increase in weight in the wet and dry atmospheres. If the compounds oxidize a t the same rate, the difference in weight increase should remain constant after the first year. If the test piece oxidizes more rapidly in the dry box, the difference in weight incrwse should dimuush. The data show, in the case of compound A, that the curves other than the overcure oxidize a t about the same rate in both the wet and dry atmospheres but the ovorcure oxidizes less in the wet atmosphere. (F)
111
~
~
~
486
INDUSTRIAL AND ENGINEERING CHEMISTRY Table 111-Change
COMP. A
B
c D
E F
OF
CURE 60 100 110 130 50 80 90 120 50 90 100 120 180 240 270 330 60 90 100 130 50
so
G
H
I
AGED4 MONTHS
TIME
90 120 70 90 100 130 90 150 180 210 30 50 60 90
$ 0 29 $ 0 32
fO.34 + O 41 li0.41 +0.22 $0.22 f0.31 $0.27 f0.27 $0.21 $0.57 $0.37 $0.41 $0.39 $0.53 f0.29 +0.38 f0.38 $0.55 $0.32 fO.29 4-0.29 $0.32 $0.22 +0.22 $0.17 $0.32 $0.38 +0.45 4-0.50 4-0.64 $0 26 $0 39 $ 0 43 $0 63
Indoor $ 0 22 $0 22 + O 24 1 0 46 io.12 +0.02 4-0.15 $0.31 +0.15 +0.24 $0.28 $0.62 $0.30 f0.30 f0.23 -I-0.33 +O.l5 4-0.17 +0.31 $0.53 $0.15 $0.15 $0.22 +0.32 +0.05 $0.02 $0.07 $0.61 $0.26 f0.43 +0.38 4-0.62 0 0 + O 02 f 0 11 + O 26
17 - 0 10 -0 05 +,~O 07 -0.29 $0.07 -0.24 $0.34 -0.24 0.0 -0.04 SO.09 -0.05 +0.05 0.0 0.0 -0.12 -0.10 +0.03 $0.10 -0.24 -O,l5 -0.17 -0.15 -0.09 -0 12 $0.07 +o. 15 $0 05 + O . 17 fO.29 f0.74 -0 13 -0 20 -0 30 $0 15
in Weight Calculated on a Basis of 100 Per C e n t Rubber AGED 12 MONTHS
Fbix'y -0
d"O"0'; 37 34 + O 41 +0.41 +0.41 +0.46 $0.51 4-0.65 10.31 +0.46 $0.60 $0.98 $0.39 +0.43 $0.39 $0.53 f0.46 $0.58 $0.63 +0.72 $0.27 +0.37 fO.29 $0.32 + o . 19 $0.19 $0.22 $0.63 $0.50 4-0.55 i-0.62 +0.76 +O 76 +O 83 +O 85 + O 89 +O +O
$0.49 $0.63 $0.63 co.90 +0.44 +0.34 $0.29 $0.61 4-0.31 $0.58 $0. 71 f1.71 $0.48 $0.53 +0.62 $1.28 +0.74 $1.01 +1.11 $1.42 $0.34 f0.39 4-0.46 f0.61 f0.46 +0.51 $0.49 4-0.90 4-0.79 $0.95 C1.33 f1.76 $0.22 4-0.52 f0.72 f1.15
Indoor
Fiz
$0.20
-0.07 + O . 32 + 0 . 2 4 $0.39 $0.34 $0.73 $0. 59 $0.07 -0.29 -0.07 -0.19 $ 0 . 10 + 0 . 0 7 $1.21 $0.65 + O . 15 -0.21 $0.37 $0.57 $0.72 $0.37 $1.46 $0.63 4-0.23 $0.05 $0.30 $0.23 f O . 32 $ 0 . 19 + 0 . 5 9 $0.23 $0.29 4-0. 19 +0.55 4-0.38 4-0.72 $0.65 $1.23 $0.91 $0.12 -0.17 $ 0 . 17 0.0 $0.27 f O . 0 2 4-0.49 $0. 17 $0.41 $0.27 4-0.44 $0.51 4-0.56 $0.75 f 1 . 5 3 +1.22 4-0.40 4-0.14 $0.74 $0.55 +0.86 4-0.83 + 1 . 3 3 $1.83
.... .... .... ....
.... .... .... ....
Since the undercure of compound A, for example, oxidized a t about the same rate in the wet and dry atmospheres, but deteriorated in physical properties to a greater extent in the wet atmosphere as shown by tensile strength tests, it must be concluded that oxidation alone will not explain the aging of rubber. Without these data one might think that moisture in rubber influenced a g h g because it simultaneously influenced oxidation to a corresponding degree. Boggs and BlakeEhave shown that the absorption of water causes rubber to swell. This swelling puts the rubber under continuous strain, which may cause it to age more rapidly from a physical viewpoint than would be expected from the data on the weight increase due to oxygen absorption; this difference in physical-chemical aging is especially large in undercured stocks, which swell to the greatest extent. Since moisture retards the oxidation of an overcured slab, why does it not greatly reduce the oxidation of the undercured compound, where much more moisture is absorbed? The answer is probably linked closely with the structure of rubber. Compounds B, C, and F behaved similarly to A. I n the case of D the results were erratic. Moisture was more effective in reducing oxidation in compound G than any other, and the physical results for this compound were especially good in the wet atmosphere. The large increases in weight of E and H in the wet atmosphere year after year were probably due to hydration of the clay and magnesia. (7) The change in weight of the rubber slabs in the dry box proved to be a good criterion of the deterioration during aging (Curve 24 and Table V). This conclusion would be less reliable where moisture was present, because of the influence of varying amounts of hygroscopic material. When the test slabs in the dry box have increased 1 per cent in weight calculated on the rubber in the compound, they have lost about 50 per cent of their reenforcing properties, and they continue to lose about 50 per cent for each increase of 1 per cent in weight. 8
IND. ENG.CEEM.,18, 224 (1926).
Vol. 20, No. 5
-I
AGED24 MONTHS
Et:
Indoor
$1.73 $1.56 +1.83 $2.00 $1.95 +2.57 +2.69 $3.00 $1.49 +1.92 $2.37 $3.09
f0.78 $1.12 +1.17 $1.78 4-0.44 +0.56 $0.46 $1.40 $0.52 $1.71 f1.99 +2.98 $1.68 $0.74 $1.90 4-0.85 +2.16 4-1.27 4-2.66 + 3 . 3 3 $2.37 $1 39 $3.11 + 1 93 f 3 . 2 0 4-1.95 $3.59 $2.33 +1.22 f 0 . 5 6 +1.46 f O . 8 1 + 1 . 3 2 $0.95 +1.68 $1.29 $1.10 f l . 0 5 +1.29 4-1.14 $1.29 $1.12 $ 2 . 0 0 $1.83 +2.21 +1.33
f 2 . 2 6 4-2.02
f2.62 4-4.17 4-1.33 +1.41 4-1.52 +1.85
$2.91 +3.26 iO.26 $0.76 +1.11 $2.13
E . E':
SO.20 $0.66 +0.81 $1.20 +O.OZ
$0.07 $0.31 $1.45 fO.27 +1.16 $1.37 $2.38 +O.Zl 4-0.42 $0.84 11.78 $0.82 + 1 39 $1 66 $2 98 +0.12 f0.29 f0.34 $0.93 +1.05 f0.98 $1.27 $1.43 fO.60 $1.07 $1 50 12.53
'.. .. ....
....
. . ..
AGED48 MONTHS
., .. .... . . ..
4-2.59
f1.88 +1.88 $2.02 $2.15 +2.32 $2.20 +2.30 +2.93 $1.52 $2.06 $2.43 i-3.23 $1.74 11.79 +2.27 +3.30 4-3.15 +5.47 $5.30 $6.77 $1.39 $1.59 +1.56 f1.83 f1.24 $1.39 $1.49 +2.49 $4.45 $4.19 f4.77 4-6.59 $1.41 $1.94 +1.83 +2.44
:::; -0.27 +0.59 $0.61 $1.46 -0.12 -0.05 $0.22 $1.31 0.0 $2.11 4-2.56 $4.12 fO.90 $2.11 $2.83 $4.85 $1.81 4-2.34 $2.29 $2.72 $0.07 $0.02 $0.54 $1.37 f1.07 $1.39 f1.46 $2.26 $0.55 f2.43 f3.72 f4.23
....
.... .... ....
Indoor
Dry box
Wet box
$0.27 4-1.39 $1.46 $1.93
+0.34 $0.98 +1.12 f2.09 $0.22 $ 0 . 56 $1.62 $3.61 +0.24 $1.34 +Z.Ol +3.52 $0.62 $3.89
+2.05 +2.34 +2.54 +3.00 $2.37 +2.52 +2.76 +3.87 $1.80 $2.70 +3.05 +3.94 +2.40 +2.66 $7.86 +8.89 +6.12 $8.74 +8.55 $8.89
$0.02
$0.48 $0.82 +2.45 +0.43 +1.86 +2.18 $4.27 $0.41 f1.18 12.32 4-7.31 $2.26 +3.40 $3.61 +4.00 +0.15 4-0.41 +O. 68 +1.58 $1.73 $1.90 f2.41 +4.38 +0.95 +1.83 +2.71 $4.52
.... .... ....
+ 3 : 92 +2.63 +2.75 f3.23 $3.44 $0.15 +0.68 +0.71 +1.20 f1.15
+2.00
$3.46 +4.87 4-0.74 +1.67 +KO7 4-5.88
....
....
.... ....
....
$1.56
+1.80 $1.71 $2.20 f1.65 +1.92 +2.21 +3.63
56.14
$6.81 +7.48 $9.60 4-1.41 $1.74 $2.09 f3.02
11-Aging of Rubber under Strain
Light has a decided deteriorating effect on rubber under strain. Even light that has passed through a window pane, which cuts out the shorter wave lengths, causes cracking. Since this kind of aging is believed to be different from aging "on the shelf," tensile test pieces of each cure were fastened to boards under initial strains of 50 and 200 per Table IV-Difference in Wei$ht Increase i n Wet a n d Dry Atmospheres, Calculated on a Basis of 100 Parts of Rubber (Figures in per cent) COMP.
A
B
C
D
E F
G
H
I
TIME OF
CURE Min. 60 100 110 130
50 80 90 120 50 90 100 120 180 240 270 330 60 90 100 130 50 80 90 120 70 90 100 130 90 150 180 210 30 50 60 90
TIMEOF AGING-MONTHS 4
8
12
18
0.54 0.44 0.46 0.34 0.70 0.39 0.75 0.31 0.55 0.46 0.64 0.89 0.44 0.38 0.39 0.53 0.58 0.68 0.60 0.62 0.51 0.52 0.46 0.47 0.38 0.31 0.15 0.48 0.45 0.38 0.33 0.02 0.89 1.03 1.15 0.74
1.15 0.86 0.85 0.76 1.55 1.84 1.70 0.92 1.16 0.96 1.32 1.79 1.18 0.97 1.27 1.59 1.35 1.63 1.45 1.59 0.96 0.97 0.90 0.88 0.69 0.49 0.22 0.61 1.26 1.07 1.00 1.30 1.35 1.48 1.65 0.95
1.80 1.32 1.49 1.41 2.24 2.76 2.62 1.79 1.70 1.55 2.00 2.46 1.63 1.67 1.97 2.43 2.18 2.73 2.55 2.68 1.39 1.46 1.30 1.51 0.83 0.78 0.54 0.78 2.07 1.71 1.79 2.34
1.81 1.39 1.44 1.22 2.54 2.41 2.09 1.25 1.65 1.29 1.65 2.01 1.75 1.44 1.71 2.09 2.20 3.14 2.77 2.85 1.90 1.41 1.39 1.42 0.73
.. .. .. ..
0.52
0.22 0.17 3.31 2.47 2.55 2.95
.. .. .. ..
24
36
1.81 1.64 1.34 1.22 1.24 1.34 1.05 0.88 2.42 2.01 2.13 2.74 1.48 1.07 0.85 0.43 1.62 1.21 1.34 1.14 1.46 1.06 0.70 1.64 1.60 1.29 0.81 -0.87 1 02 1:24 2:32 1.75 2.13 3.93 5.342.92 4.83 4.20 4.99 1.49 1.42 1.03 1.10 1.27 1.07 1.29 1.05 0.49 0.63 0.41 0.21 -0.09 -0.54 -0.61 -0.16 4.07 4.59 3.28 4.43 3.37 4.10 3.21 3.60
.... ....
.. ..... .
48
1.71 1.36 1.42 0.91 2.15 1.96 1.14 0.26 1.56 1.36 1.04 0.42 1.78 -1.23 4197 3.49 5.99 5.32 5.45 1.41 1.12 1.00 1.00 0.50 -0.08 -1.25 -1.24 5.40 5.14 4.41 3.72
.. .. ....
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1928
'
487
I
CURVE 3
S A M P L E A - I10 ' CURE
320
2
'
0
I50
f
CURVE 5 SAMPLE 8 - 5 0 ! CURE
1,
I
600
750
I
CURVP 6 SAMPLE B - 8 O ' C U R C
I
226
2800
2400
2000
1600
800
400 E l ONGA T/ON
0 450
300
600
750
0
I50
300
450
Nof&In Curves 1 to 24 the heavy line shows stress-strain curve before aging. W is stress-strain curve obtained after 4 years' storage in a moist atmosphere. D is stress-strain curve obtained after 4 years' storage in a dry atmospheie. I is stress-strain curve obtained after 4 years' storage indoors; and 0 is stress-strain curve obtained after 4 years' storage outdoors.
cent elongation, and one of the sets of test pieces was placed on the roof in the sunlight and the other in the dark in a loft over the laboratory. The test pieces that were exposed to the sun and weather showed a considerable variation in resistance to light. Compound D, which contained no pigment, failed within a few hours and the opening of the cracks could be watched. Compound C, with only zinc oxide for activation, showed slightly greater resistance to the light, and the other compounds showed a marked superiority, especially compound F which contained paraffin. The value of paraffin in this respect is well recognized by rubber technologists. Qualitative observations with the non-pigmented compound (D) showed that the destruction was especially rapid on a hazy day-this may be attributed to humidity. It
seems probable that accelerated aging tests where the rubber compounds have been exposed to light have not given s a c i e n t attention to the moisture content of the rubber and the atmosphere. At the end of about ll/a years all the samples except a few of F compound had broken or had assumed a set so high that the stress had become practically zero. The exact number of days for each piece to break is given in Table VI. The samples were left in this condition for 4 years and 10 months, when the test pieces that had not failed were tested for tensile properties. The results of the tests are given in Table VII. The surface of these test pieces was checked and a chalky layer 0.01 inch (0.25 mm.) deep was found. The cross section of the solid center of the test piece that had been stretched to 200 per cent clongation was about 0.08 by 0.04 inch (2.03 by 1.02 mm.). A tensile force of 22/9 pounds (1.2 kg.) was required to break it, which calculates to 835 pounds per square inch (59 kg. per sq. cm.). This is a remarkable figure considering the aging exposure
of Change i n Weight to Decrease in Area u n d e r Stress-Strain Curve of Test Pieces Stored i n a Dry Atmosphere
Table V-Relation
~
AFTER2 YEARS'AGING
CURE A
B
C
D
E
F
AFTER4 G
H
1
1
A
B
C
YEARS'
D
AGING
E
F
68 80 89 94 2.63 2.75 3.23 3.44
G
H
I
-40 8 32 66
4 78 86 86
- 7 -40 68 - 3 71 13 93 89
0.15 0.68 0.71 1.20
1.15 2.00 3.46 4.87
PER CENT DECREASE I N AREA UNDER STRESS-STRAIN CURVE
Under Intermediate Intermediate Over
S
29 43 61
-12 5 33 68
-10 52 72 96
9 71 90 98
35 64 70 86
-16
3 23 58
-10 44 69 81
-4 41 63 81
-36 17 10 70
7 40 51 63
-13 20 47 87
-25
66
93 99
PER CENT INCREASE IN WEIGHT CALCULATED O N BASIS OF 100%
Under 0 . 0 7 -0.10 -0.10 Intermediate 0 . 5 4 0 . 0 7 0.72 Intermediate 0 . 6 8 0 . 8 2 0.97 Over '1.10 2.08 1.59
0.14 0.98 1.22 2.06
1.40 -0.10 1.54 0.56 2.38 0.29 2.57 0.54
0.61 0.98 1.58 2.65
0.38 0.91 1.40 3.38
,.
.. 2:59
0.34 0.98 1.12 2.09
0.22 0.56 1.62 3.61
0.24 1.34 2.01 3.52
46 98 99 99 RUBBER
0.62 3.89 3:92
0.74 1.07 3.07 5.88
.. .. .. ..
INDUSTRIAL AND ENGINEERING CHEMISTRY
more than the thicker ends and in turn stretched further. The set on release was accordingly in many cases greater than the original amount of elongation.
254
726 AMPLE
C
- 50'
CURP
SAMPLE
C
-
Vol. 20,No. 5
90' CUP€
'07
111-Comparative Aging of Cured and Uncured Rubber
/60
The uncured stock left after making the slabs required in the preceding part of this work was wrapped in holland cloth and stored in a dark room in the laboratory for 18 and 36 months before curing. The results of this storage are given h Table VIII. On comparing tests made on slabs cured up a t the time of milling with similar stocks stored in the uncured state for 18 and 36 months and then cured, the following facts are noted:
,d/
/I3
85 %
'8
~
and speaks well for the protection afforded by the white pigment and the p a r a h . Tener, Smith, and Holtg have stated that dark-colored compounds are more resistant to sunlight than light-colored compounds. However, they used whiting in their work, whereas the writers used zinc oxide. Stutz'O has shown that there is a considerable difference in the opacity of these pigments to the short wave lengths of light, zinc oxide being entirely opaque at the shorter wave lengths whereas whiting is quite transparent. He has also shown that zinc oxide protects a paint film against chalking while pigments that are transparent a t the shorter wave lengths fail to protect. The protection given to rubber by zinc oxide is the same as that given to a paint vehicle. The color of the rubber compound in itself is of secondary importance in resistance t o sunlight. The important factor is the opacity of the pigment to the particular wave lengths of light that are injurious. Jecuscol' has made a good start along this line in studying the deterioration of rubber caused by light through investigating the aging effect of different wave lengths. This kind of deterioration has been a much more important problem to the paint chemist than to the rubber chemist, as the paint chemist is largely interested in surfaces. He has done a great deal of work on accelerated tests and we can profit by studying some of his methods.'* Only a few test pieces which were exposed in the dark were broken after 3l/* years. Those that did break were the ones that had been given an initial strain of 200 per cent elongation. Whereas in sunlight overcures checked 28rn more rapidly than undercures, in the dark the reverse was the case. This was 2400 t only a surface effect, however, and after 31/2 years of strain the undercures generally tested stronger than the overcures. The distance between the marks on the tensile-strength test pieces, which were originally 3.81 cm. (1.5 inches) and 7.62 cm. (3 inches) in the stressed position, increased as time went on. Owing to the greater strain in the thin section of the test piece, this part weakened 10
Bur. Standards, Tech. Paper 842. J . Franklin Inst., 200, 87 (1925).
11
IND. END. CHRM.,18, 420 (1926).
12
Nelson and Schmutz, Ibid., 18, 1222 (1926).
(1) The optimum for the samples cured and tested 24 and 48 hours after milling was generally one of the two intermediate cures. (2) After aging in the cured state the optimum came a t the shortest cure, which is in line with Required for Failure of a Test Piece Exposed to Weather under Strain
Table VI-Days
50% ELONGATION
COXP.
A B
C
60a 265 274 353 50 285 353 366 50
100
308 344 275 80
275 501 285 90
110
288 294 285 90
304 281 305 100
130
255 304 264 120 241 304 229 I20 299
... ... ... ... ... ... ... ... ... . 245 .. 180 240 270 330 ............ . . . . . . . . . . . . . . . . . . 602 635
D
60
E
90
100
130
.,.
658
...... . . . . . . 635
391
635 314
50
80
90
120
90
100 286 635 305
258 262 286
...
789
180
210
740
...
200% ELONGATION
1
60
100
230 338 224 50
70 222 224 210
208 209 209
90
150
180
60
90 :(?)
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 366 . . . . . . . . . . . . . . . . . 635
G
70 233 305 314 90
H
740
130
...
..... . .602. . . .. .. . .. .. .. 30
I
5
50
60
130
4 240 31 265 1 90 210 215 229 80
50
236 239 243 50 213 209 229 180 221 221 265
219 222 221 80 224 217 231
F
635 339 635 150
110
219 224 224 120 90 225 217 227 212 219 214 100 120 6 5 0 ) 73(?) 41 67 1 4 270 330 37 1 1 2 1 1 100 130 211 351 19s 212 203 90 120 602 217 358 211 602 212 100 130 206 208 209 200 207 205
260 265 255
224 246 238 30
239 232 538
90
216 211 211
184 198 200 50
227 231
228
Numbers in italics refer to minutes' cure at 40 pounds.
...
198 67
200 60
214 205 209
210
49 43 31 90
198 200 200
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1928 2300
'
C U R V P 13
Properties after Straining T e s t Pieces in L i g h t for 4 Years a n d 10 M o n t h s
Table VII-Physical
1
C U R V E 14 SAMPLE E -13o'cuRE
S A M P L E E -6O'CURE
/
$
THICKNESS AFTER
OB
RELEASEOF TENSION LOAD SET
SAMPLE
489
Shoulder Middle of tensile of tensile
141,50c.
STRETCHED 60 PER
F
50 80 90 120
85 87 87 85
F
50 80 90 120
235
0.384 0.368 0.351 0.353
T~~~~~~ ELONGATION
B&K
B RAT EAKb
CENT
0,312 0.277 0,251 0.269
4.4 3.5 3.0 3.5
18 15 13 15
255 210 190 220
460 345 280 185
70
400
STRETCHED 100 PER CBXT
a
b 226
/9 7
i69
/6I
/13
85
56
0,366
... ... ...
... .
I
.
0,183 ...
...
...
...
~YOELONGATIGIII 450 600
750 0
300
2800
'
I
CURVE
/B
'
1
SAMPLE H - OO-CURE
CUaM
/SO
300
150
600
150
I
I
I
/7
SAMPLE G - I W
CUQf
I
ELONGATiON
0
150
0
I50
300
450
600
750
450
600
750
the statement of Geer and Evansa that overcuring is a major factor in aging. (3) The optimum in the case of the stock stored uncured occurred a t the longest cure; the very reverse to the situation for storage in the cured state. Storage has retarded the ccre.
One of the stocks (G) deteriorated very appreciably in the uncured state. The reason for this peculiar behavior is an open question, but it has been generally observed that master batches of crude rubber with certain accelerators become very soft and sticky in a short time. The most probable cause for the retardation in cure is the formation of sulfurous and sulfuric acid due to the oxidation of some of the sulfur in the compound. A second possibility is that the organic accelerator decomposes during storage. The fact that this retardation occurred in the case of compounds D and H, which contain no added organic accelerator, is partial negative evidence in favor of the first explanation.
. . . . .
.....
. . . . . . . . . . . . . . . . . .
Based on initial width and thickness. Based on marks originally 2.54 cm.apart.
20
/50
5
. . . . .
Some clay and carbon black stocks show a rapid retardation that has been attributed to adsorption of the accelerator by the pigment. Since the retardation is pronounced in the case of C, which contains practically no pigment, the adsorp tion explanation does not apply in this case. The extent of the retardation during the first 18 months is much greater than during the second 18 months. The retardation of cure on storage of the uncured stock makes the practice of aging stocks for two or more weeks between milling and processing rather questionable. Certainly, if this is done it will be quite advantageous to keep this period constant.
*
0
1.2
300
INDUSTRIAL AND ENGINEERING CHEMISTRY
490 3600
1
'
254
Sbrnde A
zoo
'
0
I
8
24
I6
It i s a l s o i m portant to know 0 8 I6 24 32 40 whether the reenforcing properties have been affected by storage of the uncured stock. If not, it should be possible to bring the uncured stock back to practically its initial condition by adding organic accelerator. The data in Table IX show that, although the initial properties can be nearly restored by this method, the stiffness of the stock especially a t the higher cures, is greater, and the percentage elongation at break is less. It was also noted that the aged compound was sticky, and the added accelerator had to be milled in at low temperatures to prevent the stock from sticking to the back roll of the mill. I n the writers' opinion the deterioration in physical properties, though important, is secondary to the retardation of cure. Conclusions
PABTI-Compounded cured rubber stored in a wet box has a softer stress-strain curve than when stored in a dry Table VIII-Relative
COMP.
AT
40 LBS.
Lbs./sq.
A B C
D
E F G
H I
Min. 60 100 110 130 50 80 90 120 50 90 100 120 180 240 270 330 60 90 100 130 50 80 90 120 70 90 100 130 90 150 180 210 30 50 60 90
(fl.
Load at 300% elong.
2940 3020 2715 2725 2830 3040 8095 3050 2155 3400 3165 3135 2165 2470 2800 2425 2115 2210 2175 2650 2500 2880 3180 3190 1645 2075 1955 1475 2260 2690 2765 2380 2580 3440 3495 3690
% .. 640 630 585 575 650 610 620 590 805 730 735 685 1020 980 945 890 610 635 650 670 635 630 635 635 530 560 520 420 580 640 600 600 625 645 630 645
40
48
box. Ifdriedafter a long exposure to a moist atmosphere the tensile strengths increase by 21 to 28 kg. per sq. cm. (300 to 400 pounds per square inch). On a basis of tensile tests, overcured rubber deteriorates faster in a dry atmosphere than in a wet atmosphere and the reverse is the case for undercures. An undercure will oxidize a t about the same rate in a wet as in a dry atmosphere, but an overcure will oxidize more rapidly in the dry atmosphere as shown by changes in weight. The change in weight of samples stored in a dry box, which excludes moisture changes, proved a good criterion of their deterioration. Each increase of 1 per cent in weight calculated on the rubber content showed a corresponding decrease in reenforcing properties of 50 per cent. PART11-The ability of zinc oxide and para& to protect rubber under strain against sunlight was shown and it was suggested that humidity was a factor. The protection that pigments offer against sun-cracking is the same in both paint
Aging of Cured a n d Uncured ComDounds
I
CURED
Tensile ,trength
32
AFTER18 MONTHS'AGINGINDOORS
ORIGINAL
TIME CURE
Vol. 20, N o . 5
Tensile strength
E1ong'
L",d 300% elong.
1
AFTER36 MONTHS'ACINQ
UNCURED
Load at 300% elonn.
Tensile strength
LbsJsq. rfl.
615 730 815 865 495 705 675 800 140 220 260 305 100 120 130 150 660 680 715 795 455 645 675 700 640 720 810 1085 625 655 820 795 470 625 740 770
3100 2485 2370 1915 3090 2490 2370 1375 2185 2050 1865 265 2020 1590 1785 275 2175 1865 1670 1290 2615 3005 3135 2835 1520 1395 1630 900 2435 2665 2335 1376 3200 3050 2780 2090
640 575 530 475 620 530 525 350 780 600 555 195
490
m ..n . 540 510 370 605 610 615 595 535 440 460 250 635 595 570 435 590 595 545 475
595 960 1000 1050 760 1000 10.45 1195 155 300 365
..
100 100 90 125 840 990 935 1095 655 820 930 745 645 865 980
..
580 825 925 980 700 730 960 990
2165 2910 2945 3125
I
900 2040 2250 2595
590 620 610 610 570 630 605 605 830 710 735 735 990 985 890 880 575 635 685 670 485 580 595 620
505 685 720 785 470 600 710 765 90 145 190 135
...
35 85 95 540 710 700 815 310 505 470 655
a
1
I
1735 2130 2270 2335 1370 1960 2265 2565
The aged uncured stock was very tacky, stuck to cloth, and cured porous.
535 560 550 580 515 555 540 600
UNCURED
CURED
605 730 820 840 345 500 695 680
Load at elong. 300%
Tensile strength tbs./sq. rn.
%
2655 1905 1920 1460 2675 2285 1910 855 1450 610 435 335 1840 1470 190 245 1610 1000 890 710 2765 2645 2430 2035 1160 1150 1040 765 1700 1745 1875 700 2985 3060 2405 1615
565 455 475 355 580 485 430 215 690 415 370 260 855 760 410 120 435 245 195 160 680 560 550 480 435 345 295 170 510 475 460 170 605 605 505 460
Load Tensile at strength *long. 300% elone.
Lbs:/sq rn.
775 1090 1050 1215 815 1140 1130 ~~~
..
135 345 290
..
2150 2435 2415 2530 1505 2215 2135 2295 1000 2110 1850 2765
595 610 585 785 665 760 590 825 560 495 575 760 570 705 590 655 840 75 775 185 735 180 740 230 Stock lost
1670 2105 2230 2460 1595 1975 2130 2150
480 810 535 980 560 985 695 1000 600 300 580 570 585 680 565 615
160 180 220
.. ....
1170
..
825 770 1020 1075 830 1030
a
*...
660 895 980
..
715 730 1000 1030
1355 1710 1935 2085 1510 2305 2465 2900
525 525 525 560 595 630 610 650
470 680 775 750 185 300 490 540
INDUSTRIAL A Y D ENGINEERING CHEMISTRY
May, 1928
Table IX-Effect
491
of Adding Accelerator t o Aged Uncured Rubber COMPOUND A
WHEN CURED
Of
Directly after mixing
After 67 months’ storage uncured
After 67 months’ storage uncured with the addition of 0.16% hexa calculated on rubber (1/1 original amount added)
After 67 months’ storage uncured with the addition of 0.32% hexa calculated on rubber
After 67 months’ storage uncured with the addition of 0.65% hexa calculated on rubber
Tensile
strength
Elong.
Min. 60 100 110 130 60 100 110 130
Lbs./sq. i n . 2940 3020 2715 2725 2130 2300 2520 2385
%
60 100 110 130
c :u ;;
COMPOUND C Load at 300% elonp..
-
Time of cure at 40 Ibs.
% Lbs./sp. i n .
120 50 90 100 120
Lbs./sq. in. 2155 3400 3165 3135 1440 2440 2500 2620
805 730 735 685 885 775 745 715
140 220 269 300 115 150 160 230
660 970 1000 1100
50 90 100 120
1820 2680 2890 3140
890 760 735 730
80 195 240 245
600 560 555 525
660 945 970 1085
50 90 100 120
2100 2700 3160 2560
815 655 680 620
115 275 310 370
585 550 535 515
755 1045 1140 1230
50 90 100 120
2230 2640 3240 2320
805 655 665 595
155 280 340 420
640 630 585 575 560 530 550 535
Lbs./sq. in. 615 730 815 S65 655 890 910 930
Min. 50 90
2020 2520 2495 2350
555 545 535 515
60 100 110 130
2460 2620 2610 2500
60 100 110 130
2545 2880 2880 2710
and rubber, and the large amount of work done by the paint chemist should be studied by the rubber technologist. PART111-The effect of aging unvulcanized compounded rubber is to retard the cure greatly and to lower the re&nforcing properties slightly. Unvulcanized compounded rubber that has lain around in holland cloth for several years is sticky and has to be milled on cold rolls, but after adding accelerator and vulcanizing a good product is obtained. The retardation, in this program, is not due to adsorption of accelerator by the pigment since some of these stocks are practically “pure gum” stocks.
Tensile Load at strength E l o w . 300% elonn.
100
If this program on “normal aging” were being started today, all of these nine compounds would not have been chosen; other formulas would have been used containing antioxidants that mere unknown six years ago; certain new accelerators would have been used; and hygroscopic6reclaim stocks would have been added. Acknowledgment The writers wish to acknowledge the assistance of Howard Hock, who drew the curves shown in this article and helped in making the tests.
Recent Work on the Oxidation of Cellulose’ A Review Covering Two Years J o h n L. Parsons HAMMERMILL PAPERCOMPANY, ERIE, PA.
R
ECENT progress towards the interpretation of the nature of oxidized cellulose (oxycellulose) and a satisfactory means for its estimation has been commensurate with the advance in the chemistry of the parent substance, cellulose. The chemistry of cellulose degradation products is necessarily intimately related to that of cellulose itself. While our knowledge of the latter has been developed chiefly from the viewpoint of pure organic chemistry, it is gratifying to note that greater cognizance is being taken of the fact that cellulose is a product of the plant world and fundamentally different in its physical and chemical properties from most synthetic organic compounds. Recently Sponsler and Dore’,* have postulated a theory of cellulose structure which takes into account the physical and chemical nature of the fiber. Unfortunately, relatively little is known with certainty concerning the chemical processes involved in plant synthesis; in fact our knowledge of the simpler saccharides is far from complete. Owing to the complex character of the problem relating to the study of oxidized cellulose, progress has been slow. 1 Presented as a report of the Oxycellulose Committee before the Division of Cellulose Chemistry at the 74th Meeting of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927. * Numbers in text refer to bibliography at end of article.
Preparation of Oxidized Cellulose According to Karrer,2oxycellulose presents a greater enigma than the formation of hydrocellulose because of the fact that the first is applied to cellulose when attacked under any condition by any kind of oxidant. There’are probably as many methods of preparing oxidized cellulose as there are oxidizing agents available, and these may act on the cellulose fiber in any combination of five ways: (1) depolymerization of the cellulose; (2) hydrolysis, with tendency to hydrocellulose formation; (3) oxidation; (4) swelling of the fiber; and (5) esterification. Knecht and Mueller3 observed that mercerized yarn is more easily oxidized by acid permanganate than by hypochlorites, whereas in an earlier study Knecht and Egan4reported that ordinary cotton yarn is more readily attacked by bleaching liquor and hypochlorous acid than by acid permanganate. I n the first example no difference was noted whether the yarn was mercerized with sodium hydroxide or nitric acid. When the oxidation experiments were carried out in a vacuum there was noticeably less variation in the copper number by means of which the degree of oxidation was measured. According to Clibbens and Ridge,smercerized cotton is attacked much more rapidly by hypochlorite solutions than unmercerized cotton, while unmercerized cotton
